The invention relates to processes for coating carrier materials with adhesives and to products obtainable by means of such processes. In particular it relates to processes for coating planar carriers such as, for example, fabric tapes, non-woven fabrics, paper tapes or plastic tapes.
Adhesive-coated carriers, at least planar carriers, are known per se and are widespread. Thus, adhesive-coated plastic tapes and paper tapes have for a long time been a routine aid in offices and homes. The consumer has also become aware of self-adhesive plasters, as quick wound dressings, for many decades in a variety of forms.
In many cases the carriers consist of non-occlusive material, that is to say they are highly permeable for water vapour and/or air. In many cases this is desirable, since in by far the majority of cases the more permeable a quick wound dressing is to air and moisture, the better is its skin compatibility.
However, in order to make the adhesive coating non-occlusive, inconvenient, impractical and usually highly uneconomic processes had to be used in the past.
It is known to provide the carrier with an adhesive application which is limited in surface area or interrupted by adhesive-free places. It is also known subsequently to perforate the material coated with the adhesive composition, for example with the aid of air nozzles.
However, these processes are unsuitable for producing microporous adhesive layers with really satisfactory properties.
Furthermore German Patent 15 69 901 discloses the following process for producing porous, self-adhesive tapes or sheets: a solvent-containing viscoelastic self-adhesive composition is applied onto an intermediate carrier with an adhesive-repellent surface, the temperature is rapidly increased so that the solvent or dispersant evaporates and the viscoelastic self-adhesive composition forms bubbles. After the adhesive layer is dry, the intermediate carrier coated in this manner is pressed against a porous substrate, that is to say a planar carrier. The self-adhesive composition is thus transferred to the substrate. The elevated pressure has the effect that the bubbles burst and fine pores result.
Although the adhesive layers formed in this manner are microporous, by no means all bubbles burst. As regards its theoretically expected results, the process described here thus does not function in an optimum manner.
In addition, the paper carrier used is not indefinitely reusable. The process is thus expensive.
German Offenlegungsschrift 36 06 199 describes an adhesive label having an adhesive layer whose surface has, within a region provided with the adhesive layer over an essentially continuous area, elevations and depressions. It is not porous or even microporous adhesive layers which are described, but continuous-area coatings.
The surface structuring which is described in this publication is macroscopic in nature.
Neither air nor solvent yapours from the intermediate carrier and adhesive layer are enclosed by this surface structure. Rather, it is only achieved that the adhesive layer is thicker in one place and thinner at another place, assuming an undulating surface.
It is also apparent that the self-adhesive objects conceived here are remote from any medical use, since not only do they lift easily off the substrate, but also, because they are not porous, they occlude the substrate.
German Offenlegungsschrift 27 19 779 relates to a self-adhesive, porous, air-permeable strip for use as dressing material. The pores are produced in that a blowing agent which is dissolved in the adhesive composition is heated and thereby expands. The blowing agent bubbles thus produced are intended to tear open the adhesive layer. Blowing agents are described in this publication which can be entirely dispensed with according to the present invention. In the only example, NH.sub.4 O.sub.3, which is also a component of baking powder, is specified as blowing agent. The risk of the ammonia resulting from the thermal decomposition of NH.sub.4 CO.sub.3 under-going undesirable secondary reactions, for example with the adhesive composition itself or else with the carrier is too high. In addition, the NH.sub.4 CO.sub.3 which is present as solid must be elaborately ground and uniformly distributed in the adhesive composition.
In addition, it is to be expected that the NH.sub.4 CO.sub.3 sediments in the adhesive composition, that is to say uniform blowing agent distribution is not achieved. Sedimentation is a well known unwelcome side effect of the dispersion of pulverulent solids in adhesive compositions. Zones with high solid agglomeration and zones with low solid content occur.
Although pressure treatment analogous to German Auslegeschrift 15 69 901 is neither described nor claimed, the person skilled in the art knows that the blowing agent more or less uniformly distributed in the adhesive composition--in addition to occasionally occurring pores--mainly forms closed bubbles. The latter must be opened by mechanical action.